An olefin thermoplastic elastomer obtained by dynamically heating a polymer composition that includes an ethylene copolymer and an α-olefin thermoplastic resin in the presence of a crosslinking agent, has been known. The thermoplastic elastomer does not require a vulcanization step, and can be molded by a normal thermoplastic resin molding method (e.g., injection molding, profile extrusion molding, calendering, or blow molding). However, an olefin thermoplastic elastomer exhibits inferior rubber elasticity (i.e., elastic recovery) as compared with vulcanized rubber. In order to solve this problem, attempts have been made to improve the properties of the elastomer by increasing the crosslink density of the ethylene copolymer or increasing the Mooney viscosity of the ethylene copolymer, for example.
The elastic recovery of the elastomer is improved by these methods. However, the mechanical strength of the resulting thermoplastic elastomer composition significantly decreases due to decomposition of the α-olefin thermoplastic resin, or insufficient dispersion of the α-olefin thermoplastic resin and the ethylene copolymer.
It has been proposed to add a large amount of mineral oil-based softener when dynamically heating the composition in the presence of a crosslinking agent in order to improve the fluidity of the composition. However, the mineral oil may bleed out due to insufficient mineral oil retention capability, so that the appearance of a product deteriorates.
In order to solve these problems, a thermoplastic elastomer composition obtained by dynamically heating a polymer composition that includes an olefin resin and an oil-extended ethylene copolymer including an ethylene copolymer having a specific limiting viscosity [η] and a mineral oil-based softener, in the presence of a crosslinking agent, has been proposed (see Patent Documents 1 and 2, for example).    Patent Document 1: Japanese Patent No. 3399384    Patent Document 2: JP-A-2008-144120